Among known conventional methods of noise reduction of air conditioning equipment are internal lining method of fan ducts with sound absorption materials and method using resonance.
With the internal lining method of fan ducts with sound absorption materials, inlet air is sent to a fan duct by the suction effect of the fan, but at the same time noise produced by the fan is also radiated into the fan duct. Noise is a collection of acoustic waves of various frequencies. Acoustic waves advance through the fan duct, reflecting on the duct wall. The sound absorption materials contain a lot of foam. Acoustic waves enter the sound absorption materials, while advancing through the fan duct, and cause diffuse reflection by the foam effect inside the sound absorption materials. As a result, the energy of the acoustic waves is converted into thermal energy whereby the energy level drops. In other words, the noise level drops. This explains the mechanism of noise reduction by sound absorption materials.
Because it is short-wavelength acoustic waves, however, that cause diffuse reflection inside sound absorption materials, high acoustic absorption effects are achieved mainly with high frequencies in general.
Among typical noise reduction methods is a Helmholtz resonator as an example of the method using resonance. The Helmholtz resonator is formed to include an opening inside a fan duct and space inside the resonator. With such construction, acoustic waves propagating through the fan duct enter the Helmholtz resonator where they resonate. Resonance causes the energy of acoustic waves to change to thermal energy. Thus, the noise level drops.
With the Helmholtz resonator, by the nature of the principle of resonance, the resonant wavelength of an acoustic wave is determined by the size of the entrance and the inner size of the resonator. In addition, it is only the acoustic wave whose frequency is high and near resonant frequency that is allowed to reduce the noise level.
Among other examples of the method using resonance is a noise reduction method using a perforated acoustic board that is provided with a perforated plate that is exposed on the inner surface of a duct and a back layer at the back thereof. This is a method of noise reduction by making acoustic waves resonate by a resonator that is formed by the perforated plate and the back layer. The principle and effect of this method are the same as those of the Helmholtz resonator.
According to the method using the perforated acoustic board, the frequency of absorbing sound depends on the diameter of the perforated plate, the thickness of the back layer, the open area ratio, and the board thickness. Therefore, low frequency noise can also be reduced depending on the design. This, however, requires a back layer of reasonable size. In addition, quite a large space is required for installation.
Among known conventional methods of reducing pressure pulsation of refrigeration cycle equipment or pump equipment is an expansion muffler that causes a loss of energy by means of diffuse reflection in an expanding part. The reduction effect of pressure pulsation by the expansion muffler covers broader bands. In order to further reduce the amount of pressure pulsation, however, the ratio of the diameter of the muffler must be increased at the entrance and exit of the expanding part. Good reduction in pressure pulsation thus requires quite a large space.
Japanese Unexamined Patent Publication No. 7-247905 discloses an embodiment of funning air to an air duct through a perforated plate. This is directed to reducing noise by funning air to the air duct so as to lower the temperature of the air of the air duct, thereby making acoustic waves to resonate with the perforated plate and the back layer thereof. Accordingly, this is entirely different in principles, actions and effects from the present invention.
Japanese Unexamined Patent Publication No. 8-143149 discloses an embodiment of exhausting air through a porous member of airflow resistance, which is attached to an exhaust hole. This is directed to reducing the sound of fluid jet by expanding an area to which the fluid jet blows so as to drop the fluid speed. Accordingly, this is entirely different in principles, actions and effects from the present invention, either.
The conventional noise reduction methods of air conditioning equipment thus configured are allowed only to reduce mostly high frequency noise. A problem is, therefore, that noise reduction effect cannot be expected for low frequency noise around a few hundred hertz or below that is most needed to be reduced in air conditioning equipment.
Another problem is posed with reference to the method using resonance. If noise level can be reduced in the low frequency range, the frequency band in which the noise reduction can effect is narrow. Therefore, if the rotational speed of a fan is changed by an inverter, an applied voltage or the like, the noise reduction effect can be expected only at some part of the rotational speed.
Another problem is posed with reference to quite a large space required to reduce noise level in the low frequency range. This makes it impossible to use the method for air conditioning equipment of limited size.
Another problem is posed with reference to noise generated by a fan blade or a fan that propagates to both directions, to the blow side and the suction side. In order to reduce this noise in both directions, a separate noise reduction mechanism is required on each side, which will make the structure complicated and require quite a large space for installation.
Another problem is posed with reference to space for pressure pulsation reduction. A substantial reduction in the pressure pulsation of refrigerant generated in refrigeration cycle equipment and the pressure pulsation of water or brine generated in pump equipment requires quite a large space.
The present invention is directed to solving the aforementioned problems. It is an object to provide air conditioning equipment, fan equipment, and a method of reducing noise of equipment that allow sufficient reduction effects of low frequency noise at a few hundred hertz or below.
Another object of the present invention is to provide air conditioning equipment, fan equipment, and a method of reducing noise of equipment that allow reducing low frequency noise in a wide frequency range.
Another object is to provide air conditioning equipment, fan equipment, and a method of reducing noise of equipment that do not require large space.
Another object is to provide air conditioning equipment, fan equipment, and a method of reducing noise of equipment of simple structure that use small space for installation by reducing noise in both directions, to the blow side and the suction side of a fan blade or fan based on one method of noise reduction.
Another object is to provide a low cost system that is structured so that when the rotational speed of the fan blade or fan equipment is changed, the frequency range or the acoustic pressure level in which the noise reduction can effect automatically changes in accordance with the rotational speed changed, by using a pressure difference between before and after a fan blade or fan equipment as a drive source.
Another object is to provide a method of reducing pressure pulsation that does not require large space.